Our work and that others has established the existence of a robust correlation between loss of neuronal cell cycle control and cell death. Experimental evidence suggests that that blockade of the cycle is effective at blocking neuronal cell death, and we propose that this strategy represents a promising new approach to disease therapy. We have gained insights into the mechanisms that regulate the neuronal cell cycle through our discovery of an unexpected 4- protein cell cycle suppression complex. Our findings represent an important advance, but also raise several key questions. The first is whether the properties and functions of the 4-protein complex that we have defined in vitro apply to the behavior of nerve cells in vivo. Thus, in the first Specific Aim we will determine how the properties of the complex are altered in the brains of genetic models of ataxia-telangiectasia (AT) and Alzheimer's disease (AD). We will also create new lines of transgenic mice in which selected components of the 4-protein complex are expressed in specific sub-cellular compartments from a neuron-specific promoter. We will breed these transgenes into a AT or AD background, where our model makes specific predictions about the behaviors of double mutant neurons. If these predictions prove correct, we will have identified an important new avenue for therapeutic intervention. Another question left unanswered by our work so far is how does initiation of cell cycling in a neuron cause its death. The second specific aim is to test both the microtubule associated protein tau and Cdk5 as potential mechanistic links between cell cycle and cell death. Tau is a known component of both cycle and death pathways and we hypothesize that it links the two. We will use in vitro perturbations to learn the actions of tau and its various binding partners and how they shift when cycle-related neuronal death (CRND) is triggered. We will breed a series of double mutants between our AT and AD models and a tau knockout mutation in order to test the role of tau during CRND in vivo. Other experiments will explore the option that Cdk5 is itself the link between cycle and death. The third Specific Aim will address the question of what other proteins interact with the cell cycle suppression complex. We are particularly interested in RB (retinoblastoma) as it is both a normal binding partner and an enzymatic target of members of the complex. We will expand this work further by developing assays to screen for compounds that alter Cdk5 localization and test these for potency in blocking neurodegeneration in vitro and in vivo. In the aggregate, these studies promise to open fresh avenues of approach toward the prevention of neuronal cell loss in any number of CNS diseases. PUBLIC HEALTH RELEVANCE: The work of the past two granting periods has established the existence of a strong correlation between loss of cell cycle control in a CNS neuron and cell death. We propose a focused series of in vitro and in vivo experiments to define this linkage at a molecular level, and through that knowledge speed the progress towards new and potentially valuable approaches to combat a range of neurodegenerative diseases that currently have few disease-altering interventions.